Their faces extend in elongated snouts, and two lower teeth grow upward into horny tusks, making beaked whales one of the more fanciful creatures to prowl the deep.

Although they’re the most diverse marine mammals next to dolphins, you’re not likely to ever see one. These whales are stealthy, deep-diving, and according to a new study, possibly declining.

The study, by researchers Jeffrey Moore and Jay Barlow with the Southwest Fisheries Science Center, was published last week in the online science journal PLOS ONE. It analyzed nearly two decades worth of data on the whales to estimate their numbers.

The researchers concluded that while beaked whale populations are hard to count, they’re probably dwindling, and Navy sonar and changes to ocean chemistry may be to blame.

“We have high certainty that those have been declining,” Moore said.

Navy officials said they’re reviewing the study, but said its findings are too vague to implicate its sonar operations as a cause of whale decline.

Beaked whales range from 13 to 42 feet long, with narrow, beak-like snouts that inspired their names They’re members of the group of toothed whales, along with dolphins and porpoises, but have only two teeth. Those evolved into protruding tusks that males use for fighting and luring mates.

“They’re really pretty strange looking,” he said.

Aside from dolphins, beaked whales are the most varied group of marine mammals, with at least 21 different species. Yet they’re so rare that some have been identified only by remains.

“There are a few species that have never been seen alive at sea,” Moore said. “They’re only known from a few carcasses that have washed offshore, or from a single skull or single specimen.”

The whales haunt the ocean depths, diving more than half a mile for prey and surfacing rarely, Moore said, which makes them hard to spot and even harder to study.

“As common as they might be, they are, as a whole, kind of a mysterious group,” he said.

To measure their populations, scientists relied on data from sweeping, months-long transects of the California current. Every few years since 1991, the fisheries center has embarked on research cruises to survey the coast from the Canadian to the Mexican border, extending 300 miles offshore, Moore said.

Each of those cruises sighted only a small number of beaked whales, but scientists used distance-based statistical models to calculate how many whales they didn’t spot, based on the number and location of the ones they did, Moore said.

Their estimates for populations of three main groups of beaked whales ranged widely — from about 8,000 to 25,000 — in 1991. But they suggested a downward trend by 2008, with estimate for that year ranging from about 4,000 to 15,000 for beaked whales as a whole.

Moore acknowledged that the range is wide, but said the statistical probability that they’re declining is better than 95 percent.

“It’s hard to have a really precise model of how many there are, but the way the trend model works, we’re able to make strong conclusions that ... wherever it falls in that range of uncertainty, it’s going down,” he said.

The study noted many possible reasons for decline, including hunting, entanglement in fishing nets, noise disturbance from large ships or oil and gas operations, ingestion of plastic debris and ocean ecosystem changes.

The most likely, the study concluded, are drops in deep water oxygen levels, along with Navy sonar.

As the ocean’s low oxygen zones have expanded because of warming water, the fish and small squid that beaked whales feed on may have diminished, the study stated. Meanwhile jumbo squid, which are resistant to low oxygen conditions, have expanded their ranges, and may compete with beaked whales for prey, the study stated.

Sonar can spook whales, Moore said, driving the animals away from feeding areas. In extreme instances, it can disrupt their diving behavior in a way that leaves the whales with a cetacean version of “the bends.”

Researchers have observed strandings of beaked whales following sonar testing, Moore said, and behavioral studies in the Bahamas showed that the whales flee from sonar noise, and occasionally die from its physiological effects.

Robert Gisiner, program manager for the Navy Living Marine Resources Research Program, said the Navy acknowledges that sonar poses risks to marine mammals, but said only it has been linked to strandings in only about 40 cases. The Navy has funded research by Barlow and other researchers, he said, and follows protective measures approved by the National Marine Fisheries Service, the parent agency of the Southwest Fisheries Science Center.

The study also noted, paradoxically, that some Navy sonar ranges show high densities of beaked whales. But the authors said it’s unclear if the whales are truly thriving there, or whether the areas offer high feeding opportunities but also high risks - sort of like an underwater inner city.

Gisiner, however, said the finding “suggests that even frequent sonar use in some areas does not necessarily lead to reduced beaked whale populations.”

Moore acknowledged that there’s more to be learned, and said isotopes studies may help pinpoint changes in the whales’ diets, and future marine transects will flesh out their population trends.

Gisiner said the Navy will be part of those efforts.

“The Navy is a world leader in funding marine mammal research, and will remain proactive in funding studies that shed light on how beaked whales and other marine mammals may be affected by sonar and other underwater sound,” he said.